The present invention relates generally to semiconductor optical amplifiers and, more particularly, to semiconductor optical amplifiers having an injection current with a non-uniform density across the device.
Technologies associated with the communication of information have evolved rapidly over the last several decades. Optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Semiconductor lasers and optical amplifiers are used in many aspects of optical communication systems, for example to generate optical carriers in optical transceivers and to generate optically amplified signals in optical transmission systems. Among other things, optical amplifiers are used to compensate for the attenuation of optical data signals transmitted over long distances.
There are several different types of optical amplifiers being used in today's optical communication systems. In erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers, the optical fiber itself acts as a gain medium that transfers energy from pump lasers to the optical data signal traveling therethrough. In semiconductor optical amplifiers (SOAs), an electrical current is used to pump the active region of a semiconductor device. The optical signal is input to the SOA from the optical fiber where it experiences gain due to stimulated emission as it passes through the active region of the SOA.
The electrical pumping current is typically injected via an electrode. Consider, for example, the ridge-waveguide type SOA 28 structure illustrated in the cross-section of FIG. 1(a). Therein a multi-layer active (gain) region 30 is sandwiched between the substrate layer 32 and the residual cladding layer 34. Those skilled in the art will appreciate that any gain structure can be employed as active region 30, e.g., multiple quantum well separate confinement heterostructures and/or bulk materials can be used to fabricate gain section 30. Multiple quantum wells (not shown) may be provided in gain section 30 using various materials, e.g., InAlGaAs, InGaAsP and InP, to create gain section 30 using well known techniques. Separate confinement waveguiding layers (not shown) may be provided in gain section 30 using various materials such as InGaAsP. The substrate layer 32 and residual cladding layer 34 can be formed from, for example, InP. An etch stop layer 36 is disposed on top of the residual cladding layer 34. The ridge is formed from another InP layer 38 capped by a different semiconductor layer 39, for example InGaAs, and a current-confining dielectric layer 41. A contact opening is etched in the dielectric layer 41 and a metal electrode layer 40 is disposed on top of the dielectric layer 41 such that it makes contact with the top semiconductor ridge layer 39. Current is injected via electrode 40 into the SOA 28, so that gain is applied to an optical signal passing through the active region 30. However, gain is only applied in the pumped region 42 of the active region 30. Outside of the pumped region 42, where there is no pumping current, the optical signal suffers from energy absorption as it passes through the SOA 28. The input optical power Pin injected into the SOA 28 is amplified according to Pout=G Pin, where G is the single pass gain over the length L of the SOA 28 such that G=egnet L. The net gain gnet is given by gnet=Γg−α where Γ, g, and α are the optical confinement factor, the material gain and the optical loss, respectively.
The injection electrode can be fabricated along the length of the device as a metallization layer 40 contacting the top semiconductor ridge layer 39 by first removing dielectric material 41 prior to deposition of the metallization layer 40 as seen in FIG. 1(b). Therein, the dielectric material 41 is removed between the two dotted lines below the metallization layer to create the “T-shaped” contact shown in cross-section in FIG. 1(a). The injection electrode is connected to a current source or similar device to provide the injection current to the SOA 28. As can be seen from FIG. 1(b), conventional SOAs employ injection electrodes that have a uniform surface area across the waveguide, resulting in the injection current density being uniform across the length of the SOA 28. It should be noted that although the injection current density is uniform along the length of the SOA, the carrier concentration is not necessarily uniform due to variation in optical intensity along the length of the SOA. Also, the effects of the injected current are not uniform across the length of the device. For example, on the input side of the SOA 28, the effect of the injection current density on the noise figure (NF) of the SOA is of greater concern than the effect of the injection current density on saturation power (Psat) of the SOA. On the output side of the SOA, by way of contrast, the effect of the injection current density on the Psat of the device is more important.
Accordingly, Applicants have developed SOA devices and methods which provide for control of the injection current and injection current density to, among other things, provide SOAs which have improved gain linearity, reduced crosstalk, and better efficiency through optimized current utilization.